1. Field of the Invention
The present invention relates to a liquid electrophotographic color image forming apparatus and a color image forming method and, more particularly, to a liquid electrophotographic image forming apparatus and method which prevents toner from a color image from being transferred to a developing roller.
2. Description of the Related Art
In a typical liquid electrophotographic image forming apparatus, an image is formed on a photoreceptor medium such as a photoreceptor web by using developer in which toner powder having a predetermined color and liquid carrier are mixed, and the image is printed on a sheet of print paper. To form and print an image, the image forming apparatus adopts the basic processes of discharging, charging, exposure, development, drying and transfer. Also, in the color image forming apparatus for forming a color image on a photoreceptor web, the exposure and development steps are usually repeated four times. With the trend toward high speed image forming apparatuses, four optical scanning units and four developing units are provided so that the exposure and development steps can be repeated four times during one turn of the photoreceptor web, that is, one cycle. The respective development units include developing rollers for sequentially developing a latent image formed on the photoreceptor web using developer for yellow (Y), cyan (C), magenta (M) and black (K) colors. A conventional liquid electrophotographic color image forming apparatus having the developing rollers is shown in FIG. 1.
Referring to FIG. 1, a photoreceptor web 10 is installed to be capable of circulating around a plurality of rollers 11. Devices for performing the above basic processes are sequentially installed around the photoreceptor web 10 in the direction that the photoreceptor web 10 circulates. These devices are a discharger 8, a main charger 9, four optical scanning units 12a-12d and four developing units 13a-13d alternately installed color by color, a drying unit 17, and a transfer unit 19. The units 13a-13d are provided with developing rollers 15a-15d and squeegee rollers 14, respectively. The squeegee rollers 14 remove carrier from the developer on the photoreceptor web 10. Each of the developing rollers 15a-15d is installed to be separated from the photoreceptor web 10 by the same distance, i.e., developing gap (G). Also, an auxiliary charger such as a topping corona 16 is installed near the photoreceptor web 10 downstream from each of the developing units 13a-13d. The auxiliary charger compensates for natural attenuation of the level of charging electric potential, by further charging the photoreceptor web 10.
In the operation of the conventional liquid electrophotographic color image forming apparatus, first, while the photoreceptor web 10 circulates at a constant speed, the discharger 8 removes a remaining charge component. Next, the surface of the photoreceptor web 10 is charged to a charging electric potential of about 650-700V by the main charger 9. The surface of the photoreceptor web 10 is exposed to light scanned by the optical scanning units 12a-12d which are installed in order of color under the photoreceptor web 10. An electrostatic latent image corresponding to image data for each color is formed on the sequentially exposed photoreceptor web 10. The electrostatic latent image for each color is developed using developer which is supplied through a manifold 7 while passing each of the developing units 13a-13d. About 60-70% of carrier in the developer used in the development is squeegeed by the squeegee rollers 14 and removed from the photoreceptor web 10. The remaining carrier is vaporized by the drying unit 17. Also, the toner powder in the developer used in the development is made filmy by the squeegee roller 14 and is used for forming a toner image. The toner image is finally printed on a sheet of print paper P via the transfer unit 19.
The image forming method using the developing units 13a-13d for each color is described in detail referring to an electric potential model related to the charging property.
That is, as shown in FIG. 2A, the photoreceptor web 10 is charged to a charging electric potential VCY of about 560-700V by the main charger 9. Next, the photoreceptor web 10 is primarily exposed to light scanned by the optical scanning unit 12a for a yellow (Y) color and the electric potential of the surface of the exposed photoreceptor web 10 is lowered to an exposure electric potential Ve of about 120V. An electrostatic latent image corresponding to the yellow image data is formed at a predetermined portion of the photoreceptor web 10 with electric potential lowered. Developer for the yellow color is supplied to an electrostatic latent image for the yellow color formed as above, through the developing roller 15a for the yellow color, and simultaneously, a development electric potential Vd of about 450V is applied to the developing roller 15a. The charged toner component moves to the electrostatic latent image for the yellow color due to the difference in the electric potential between the exposure electric potential Ve and the development electric potential Vd, so that an image 10a for the yellow color is formed. When the electric potential of the toner component adhering to the yellow image 10a becomes almost the same as the development electric potential Vd, the development does not continue any more, that is, balance in charge is achieved. The developed yellow image 10a becomes filmy by the squeegee roller 14 in a squeegeeing process. The filmy yellow image 10a, about 60-70% of its carrier being removed, remains on the photoreceptor web 10.
The charging electric potential of the photoreceptor web 10 naturally attenuates while passing the yellow developing unit 13a prior to entering a cyan (C) image forming step. Thus, to compensate for the attenuation in the level of the charging electric potential of the photoreceptor web 10, the topping corona 16, an auxiliary charger, further charges the photoreceptor web 10. Referring to FIG. 2B, the charging electric potential VCC of the photoreceptor web 10 which is further charged is higher than the charging electric potential VCY prior to the formation of the yellow image 10a. Also, even when the yellow image 10a is further charged, the electric potential thereof is lower than the charging electric potential VCY.
In this state, the optical scanning unit 12b for a cyan color scans light to the photoreceptor web 10 to form an electrostatic latent image for the cyan color. The development electric potential Vd is applied to the developing roller 15b, and simultaneously, developer for the cyan color is supplied to the developing roller 15b. Then, the difference in the electric potential between the development electric potential Vd and the exposure electric potential Ve causes the charged toner of the cyan developer to move to the cyan electric potential due to the difference in the electric potential so that a cyan image 10b is formed. Here, a difference in the electric potential between the yellow image 10a formed in the previous step and the cyan developing roller 15b occurs. As a result, a wash-off phenomenon where some of toner of the yellow image 10a is transferred back to the cyan developing roller 15b due to an electric field generated by the different electric potentials occurs.
Also, when an image 10b for the cyan color is formed, the photoreceptor web 10 is further charged to form an image for a magenta color. Then, an electrostatic latent image for the magenta color is formed on the photoreceptor web 10. As shown in FIG. 2C, the electric potential VCM of the photoreceptor web 10 is higher than the electric potential VCC in the previous step. Also, the electric potential levels of both the yellow image 10a and the cyan image 10b are higher than the development electric potential Vd of the developing roller 15c for the magenta color. In particular, the difference in the electric potential between the yellow image 10a and the magenta developing roller 15c is much greater due to the additional two previously performed charging steps. Accordingly, the difference in the electric potential between the yellow image 10a and the magenta developing roller 15c becomes greater than that in the step of forming the cyan image 10b. Consequently, the wash-off phenomenon is generated more severely than in the step of forming the cyan image 10b. 
Also, as shown in FIG. 2D, one more charging step is performed in forming an image 10d for a black color. The charging electric potential VCK of the further charged photoreceptor web 10 is higher than the previous charging electric potential VCM, and higher still than the original charging electric potential VCY. Furthermore, the respective yellow, cyan and magenta images 10a, 10b and 10c have different electric potentials with respect to the developing roller 15d for the black color. In this case, even when the same development gap (G) between each developing roller and the photoreceptor web 10 is maintained, as the development unit is installed closer downstream from the previous developing unit, the difference in the electric potential between the developing roller and each image becomes greater. Thus, a more severe wash-off phenomenon is generated in proportion to the difference in the electric potential from the yellow developing roller 15a to the black developing roller 15d. 
When the wash-off phenomenon is generated, some of the toner components of the respective images 10a-10c formed at an appropriate concentration by the developing rollers 15a-15c is washed off onto the respective developing rollers 15b-15d when the next color is developed. Accordingly, the images 10a-10c lack the appropriate concentration. Thus, the respective images 10a-10c become partially missing or tainted. As a result, when the color image is printed on a sheet of print paper, an incomplete print image is obtained.
Also, the toner washed off from the respective images 10a-10c is mixed with the developer contained in the respective developing units 13b-13d. Then, the developer of each of the developing units 13b-13d is contaminated, and developer contaminated beyond a predetermined limit must be replaced. Thus, the period for using the developer is shortened and, thus the cost therefor increases.
To solve the above problems, it is an object of the present invention to provide a liquid electrophotographic color image forming method and apparatus in which a strength of an electric field generated by a difference in an electric potential between an image formed on a photoreceptor web and a developing roller can be constantly maintained.
Accordingly, to achieve the above object, there is provided a liquid electrophotographic color image forming apparatus comprising a photoreceptor web which is operative to circulate, a main charger for charging a surface of the photoreceptor web to a predetermined charging electric potential, and an optical scanning unit for scanning light onto the photoreceptor web to form an electrostatic latent image. Also provided are developing rollers for yellow, cyan, magenta and black colors, sequentially installed in a direction that the photoreceptor web circulates. The developing rollers develop the electrostatic latent image using developer for each color, and auxiliary chargers for the cyan, magenta and black colors, respectively, installed downstream of each of the developing rollers, which additionally charge the photoreceptor web, the electric potential of which is lowered after development for each of the yellow, cyan and magenta colors. Development gaps provided between each of the developing rollers and the photoreceptor web are respectively defined as GY, GC, GM and GK sequentially in a direction that the photoreceptor web circulates. The development gaps are operative to restrict an increase of an intensity of an electric field at each development gap according to the additional charge, such that each of the developing rollers are installed to satisfy the condition of GYxe2x89xa6GCxe2x89xa6GMxe2x89xa6GK.
It is preferred in the present invention that the apparatus further comprises at least one light emitting body, installed between one of the developing rollers and one of the auxiliary chargers, for forcibly lowering the electric potential of the photoreceptor web after passing the developing roller.
To achieve the above object, there is provided a method of forming a color image comprising the steps of charging a photoreceptor web to a predetermined charging electric potential, and providing each of a plurality of optical scanning units which are installed in order of yellow, cyan, magenta and black colors, scanning light onto the photoreceptor web to sequentially form electrostatic latent images corresponding to the respective colors. The electrostatic latent images are sequentially developed using yellow, cyan, magenta and black developer applied from yellow, cyan, magenta and black developing rollers. The developer used for the development is squeegeed by squeegee rollers, one of which is installed downstream of each of the developing rollers. The photoreceptor web having a lowered electric potential after squeegeeing, is additionally charged by using an auxiliary charger, and the developer used for the development on the photoreceptor web is restricted from being transferred to a developing roller.
Also, it is preferred in the present invention that the step of restricting developer from being transferred back to the next developing roller further comprises a step of maintaining a magnitude of an electric field at each of the development gaps between each developing roller and the photoreceptor web within a predetermined range.
Also, it is preferred in the present invention that the step of maintaining the magnitude of an electric field further comprises the steps of installing the developing rollers such that the sizes of the development gaps between each of the yellow, cyan, magenta and black developing rollers and the photoreceptor web can be increased, and maintaining the difference in the electric potential between the photoreceptor web and each of the developing rollers at each development gap to be 150V or less, in which an increase of the difference in the electric potential at each of the development gaps according to the additional charging is compensated for by an increase in size of the development gap.